Was the red-light camera study flawed?

Was red-light camera study flawed?One expert cites lack of control group; city plans for more research

BRADLEY OLSON, Copyright 2009 Houston Chronicle

Published 6:30 am, Thursday, January 1, 2009

Photo: Michael Paulsen, Chronicle

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The intersection of Westheimer and the West Loop South is one of 50 across Houston where cameras snap pictures of red-light runners.

The intersection of Westheimer and the West Loop South is one of 50 across Houston where cameras snap pictures of red-light runners.

Photo: Michael Paulsen, Chronicle

Was the red-light camera study flawed?

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The results of a red-light camera study released this week by the city have puzzled the researchers who conducted it and others around the country.

At a ratio of 10 to 1, study after study on the effect of red-light cameras from Oxnard, Calif., to Philadelphia, Pa., (and all the way to the United Kingdom, where they have been used for years) have found that they drastically reduce crashes.

But according to the Houston study, the red-light program here had no clear impact. Accidents remained relatively flat at the lanes of intersections where cameras were installed and increased by three times in some cases in approaching lanes without cameras.

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Researchers say there are numerous possibilities, all of which need further analysis. The studies may have been conducted differently, or the cameras may have set up differently. Houston driving trends may differ compared to other cities, or the cameras may be less effective in urban areas.

One specialist from a renowned traffic research organization who reviewed the study for the Houston Chronicle said the methodology was "flawed" and has serious "limitations."

The main problem is a statistical one, said Anne McCartt, senior vice president for research at the Insurance Institute for Highway Safety. The institute has conducted several studies that were published in peer-reviewed journals on traffic research.

Because red-light cameras are known to have a spillover effect — meaning that they have been shown to impact the number of accidents at intersections where there are no cameras — robust examinations of camera programs always compare crash data with that in other cities.

It's what statisticians call a control group. Unless the study authors compare crashes at the 50 intersections where red-light cameras have been installed with other intersections in which they have not been — preferably in other cities — no conclusions can be drawn from it.

"The design of the study doesn't allow you to draw a conclusion about the effect of the cameras," McCartt said. "We believe very strongly based on lots of other good studies that red-light cameras reduce violations and crashes. ... But I don't think this study allows a person to draw a conclusion about the effects of the program."

Bryan Porter, an associate professor of psychology at Old Dominion University who has conducted red-light camera research, said he believes study authors did the best they could with the data.

"The methods are different, as they admit, which has some weakness, as well as some interesting twists on how cameras can be evaluated," Porter wrote in an e-mail.

He added that most research has shown red-light cameras are not revenue generators. Over time, as people learn and remember where they are, they either break even or cost money.

Timothy Lomax, a research engineer with Texas A&M University's Texas Transportation Institute and co-author of the Houston study, said the analysis had to be done based on the data that existed. It wasn't feasible to deploy red-light cameras at low-traffic intersections for a comparison.

"We made rational use of limited resources," he said.

Robert Stein, a Rice University political scientist who co-authored the study, said they originally proposed to examine "controllable intersections" with no cameras for the purposes of comparison, but "the city chose not to do it in that way."

Still, he said they would continue to look at other possibilities for the unusual results, such as inspecting the intersections to see how well signs indicate a point of the intersection is monitored by a red-light camera.

Michael Moore, the mayor's chief of staff, plans to seek state insurance records to test the study's hypothesis that there have been more crashes in Houston in recent years.

They also plan to check the timing of the signals to see if that could affect how well the cameras work, as well as weather and construction at the intersections.

The city paid $25,000 for the study, and Stein said the additional aspects of it should be completed by August.

Mayor Bill White said he wasn't sure why Houston's results differ from those in other areas, although he said it could be that growth from year to year has led to an increase in cars on the road and, by extension, accidents.